`check_include_file_cxx` compiles and *links*, which it should not need
In at least CMake 3.12.3 the check_include_file_cxx
function links the test-program, which isn't really needed.
For example, when checking e.g. boost/asio/io_service.hpp
the check will fail because Boost ASIO depends on Boost System, which is a linker library.
If the function only built an object file, the build (and the test) should have succeeded.
When checking for header files we're only interested about the header file itself, not about if it's possible to link programs including the header file.
The same problem probably exist for the C check_include_file
function.